1) Đk: $\left\{ \begin{array}{l} \cos x\ne 0 \\ \cos x\ne1 \end{array} \right .\Leftrightarrow \left\{ \begin{array}{l} x\ne\dfrac{\pi}{2}+k\pi \\ x\ne k2\pi\end{array} \right .$
2) $\Rightarrow 2\sin 3x=\cos 5x-\cos x$
$\Rightarrow 2\sin 2x=-2\sin3x\sin2x$
$\Rightarrow 2\sin 2x(1+\sin 3x)=0$
$\Rightarrow \left[ \begin{array}{l} \sin 2x=0 \\ \sin 3x=-1 \end{array} \right .\Rightarrow \left[ \begin{array}{l} 2x=k\pi \\ 3x=\dfrac{-\pi}{2}+k2\pi \end{array} \right .$
$\Rightarrow \left[\begin{array}{l} x=k\dfrac{\pi}{2} \\x=\dfrac{-\pi}{6}+k\dfrac{2\pi}{3} \end{array} \right .$
3)
